Recent furnaces under non-oxidizing atmosphere operation, including blast furnaces, have been scaled up and operated under severer conditions. Damaging factors toward the lining refractory materials have increased because of the trend of high-pressure operation, Pulverized Coal Injection operation and so on. Against those circumstances, it is required to suppress initial its investment and attain longer life. The life of blast furnace depends on the durability of the lining material of the side walls and bottom region of blast furnace hearth. Carbonaceous refractory materials are popularly used as the lining material for the portions of the sidewall and bottom region of blast furnace hearth. In other words, the improvement of the durability of carbonaceous refractory materials for lining material has decisive effect on the blast furnace life.
Generally, carbonaceous refractory materials are produced by adding organic binders, such as coal tar pitch, phenol resin, etc. to a carbon aggregate, such as calcined anthracite, artificial graphite and natural graphite. Then, the mixture is kneaded, formed by extrusion or compression molding, and thereafter baked in coke breeze packings. Compared with fire clay brick, carbonaceous refractory materials have a weak point because of the dissolution in molten iron. However, Carbonaceous refractory materials have been predominantly used as the lining material of blast furnace hearth. It is because carbonaceous refractory materials have high thermal conductivity and excellent slug resistance.
Currently it is well known that the causes of the damage of carbonaceous refractory materials in blast furnace are carburization dissolution into molten iron, structural destruction due to the penetration of molten iron into the pore, formation of cracks due to the penetration and reaction with alkali and zinc vapors, formation of cracks due to thermal stress and abrasion due to the flow of molten iron.
For the purpose of the improvement of the durability of carbonaceous refractory materials, therefore, many proposals have been made about the composition of carbonaceous refractory materials, manufacturing method, application method, etc., and they are actually applied the actual operation. For example, the present inventor has disclosed in Japanese Patent Publication No. Sho-56-18559 a carbonaceous refractory material for blast furnace, in which metal oxides, such as α-alumina, zircon and magnesia, are mixed into the main raw materials of the carbon aggregate to reduce the carburization dissolution rate into molten iron.
Also, the present inventor has disclosed in Japanese Patent Publication No. Sho 58-43350 a method for producing a carbonaceous refractory material for blast furnace, in which metallic silicon particles are mixed into the main raw materials of carbon aggregate to generate a whisker-like silicon compounds in the pore of the carbonaceous refractory material during the baking process, so that pores with a diameter of 1 μm or more, into which molten iron can infiltrate, to be reduced. As the results, the penetration of molten iron and infiltration of reactive gas into the carbonaceous refractory material are reduced.
Japanese Patent Laid-open No. Hei-7-172907 has disclosed a carbon-containing refractory material with improved oxidation resistance and corrosion resistance, which is produced by adding titanium carbide powder to a mixture of a carbonaceous material and an alumina. This carbon-containing refractory material forms an Al2o3/TiO2-system compounds after oxidizing reaction at high temperature, and its structure becomes dense.
Japanese Patent No. 2747734 has disclosed a carbon-containing refractory material with high corrosion resistance and oxidation resistance, which contains a carbide material, for example titanium carbide, as an antioxidant in addition to a carbon and a refractory oxide materials.
Furthermore, the inventor has disclosed in Japanese Patent Laid-open No. Hei-8-81706 a method for producing a large carbonaceous refractory material for blast furnace, which has high thermal conductivity, a little carburization dissolution property and a small pore diameter. In this Laid-open Japanese patent, artificial graphite and natural graphite, which have high thermal conductivity, are selected as main raw materials of the carbonaceous refractory materials to increase thermal conductivity of the product.
With such various countermeasures described above, the durability of carbonaceous refractory materials has been improved. However, the reduction of a carburization dissolution rate of a carbonaceous refractory material retaining high thermal conductivity and slug resistance is still limited as long as main raw material is carbon aggregate.
As described in the Japanese Patent Publication No. Sho-56-18559, the addition of metal oxides such as α-alumina is clearly effective only for reducing of the carburization dissolution rate. Namely, it is possible to reduce the carburization dissolution rate extremely by adding a large amount of metal oxides. However, the slug resistance and thermal conductivity decreases under a higher content of metal oxides.
Further, the refractory material described in Japanese Patent Laid-open No. Hei-7-172907 is insufficient in view of wettability with molten iron after oxidization at high temperature, because the refractory material does no contain metallic titanium, titanium carbide or titanium nitride. Hence, such refractory material does not have excellent corrosion resistance property. Still further, the refractory material described in Japanese Patent No. 2747734 contains 30% or less carbon. Moreover the patent does not includes any description that titanium carbide can improve the wettability with molten iron. Thus, the refractory material in this invention is based on a different technical concept from the present invention.
Additionally, one specific problem of refractory materials at the bottom of blast furnace hearth is abrasion of the inner-lined carbonaceous refractory material due to the flow of melted pig iron. In other words, iron discharge operation generates a circular flow of melted pig iron at the bottom of blast furnace. It is known that carbonaceous refractory materials in regions along the circular flow are abraded seriously, compared with other regions. Because carbonaceous refractory materials which consist of carbon aggregates as its main raw materials in particular are not wettable with melted pig iron and so they can hardly generate a protective layer on the surfaces. Therefore, a fresh surface is continuously left in contact with melted pig iron. Then, the carbonaceous refractory materials are gradually abraded by the flow of melted pig iron.
The following propositions have been made to prevent such abrasion of inner-lined carbonaceous refractory materials because of the circular flow. Japanese Patent Laid-open No. Hei-10-298623 proposes a blast furnace bottom structure and a method of operating a blast furnace, in which the blast furnace has tap holes at different level of altitude, upper and lower tap holes. Monitoring the temperature distribution in the lining material of the bottom, tap operation in upper and lower tap holes are controlled to float DEAD-MAN formed on the blast furnace bottom, and then such circular flow could be eliminated by allowing melted pig iron to move on the overall furnace bottom.
Other Japanese Patent Laid-open No. Hei-9-41009 proposes a method for preventing direct contact between carbonaceous refractory material and melted pig iron by charging a TiO2 source into a blast furnace and efficiently precipitating a Ti compound-containing protective layer with a high melting point on the bottom. However, the protective layer with a high melting point can not be fixed on the bottom of blast furnace because the protective layer with a high melting point and the carbonaceous refractory material cannot react each other or are never wetted with or bound to each other. Thus, the method can not prevent the protective layer to be washed away.
As described above, the durability of a carbonaceous refractory material can be improved by reducing the carburization dissolution rate and by allowing the refractory material to be wettable with melted pig iron. According to the conventional methods, however, the carburization dissolution rate cannot be reduced while the thermal conductivity and slug resistance are retained. Further, any carbonaceous refractory material wettable with melted pig iron has not been disclosed yet.
The present invention has been proposed to overcome the problems of the related art described above. The objective of the invention is to provide a carbonaceous refractory material with a reduced carburization dissolution rate together with attaining wettability with molten iron, particularly melted pig iron, while the carbonaceous refractory material retains thermal conductivity and slug resistance, and a method for producing the carbonaceous refractory material.